There are several known processes for splitting polyurethane foam waste by hydrolytic degradation into a mixture of the starting polyol and the diamine on which the diisocyanate used in the production of the foam is based. Before it can be reused in a production process, this mixture of hydrolysates must be split up into the polyol component and the diamine component.
A commercially workable separation process should be economic, low on investment costs and able to recover the starting materials in a form in which they can be reused without any need for expensive purification. Relative to the difficult and particularly sensitive flexible polyurethane foam formulations, this applies in particular to the quality of polyether polyols which are to be returned to the foaming process after recovery from hydrolysates.
To date, distillation and liquid extraction processes in various modifications have mainly been used for separation of the mixture of hydrolysates. It is possible by distillation to recover the diamine components in a high yield and in a highly pure form. However, the polyether which accumulates as distillation residue cannot be used in its existing form for direct refoaming (Environmental Science and Technology, Vol. 8, No. 2, February, 1974, page 138).
In the liquid extraction process, as described for example in German Auslegeschrift No. 2,207,379, the hydrolysate is taken up in toluene or another organic solvent and the diamine is extracted with dilute aqueous hydrochloric acid. The limited solubility of the diamine hydrochloride necessitates a considerable volume of dilute acid which must be removed from the extract by distillation. In addition, the stabilizers and emulsifiers used in the foam production process seriously complicate separation of the organic and aqueous phases.
U.S. Pat. No. 4,035,314 describes the recovery of polyurethane starting materials by the hydrolysis of composite plastics materials of the type used in motor vehicles. After the polyurethane hydrolysate (consisting essentially of polytetrahyrofuran and diamine) has been separated off from the nonhydrolyzable plastics by means of an organic solvent, hydrogen chloride gas is introduced into the hydrolysate solution until no more amine hydrochloride precipitates. The amine hydrochloride is filtered off and the solvent and excess hydrogen chloride is distilled off, to leave polytetrahydrofuran ether regenerate. The regenerate can then be reacted with tolylene diisocyanate to form a prepolymer which, blended with pure prepolymer, may be hardened with 4,4'-methylene dianiline to form an elastomer. C.sub.2 -/C.sub.3 -polyethers used for the production of foams cannot be quantitatively separated from the diamine in accordance with U.S. Pat. No. 4,035,314. C.sub.2 -/C.sub.3 -regenerated polyethers which cannot be used for foaming are obtained.
German Offenlegungsschrift No. 2,854,940 describes a separation process which gives satisfactorily foamable, substantially amine-free C.sub.2 -/C.sub.3 -regenerated polyethers and which is characterized in that precipitation of the amine hydrochloride is carried out in several steps. In each step, the precipitated amine salt is separated from the reaction solution before any more hydrogen chloride is introduced and the amine hydrochloride is only quantitatively precipitated using excess hydrogen chloride when the amine group content of the polyether is below 1.0% by weight. It is possible in this way to effectively suppress the formation of troublesome quantities of polyethersoluble amine dihydrochloride which is observed where relatively high concentrations of amine are present, as is the case, for example, when the process according to U.S. Pat. No. 4,035,314 is applied to a polyurethane foam hydrolysate. German Offenlegungsschrift No. 2,854,940 naturally requires exact quantitative control of the amount of HCl introduced into the hydrolysate solution and analytical control of the amine content in the individual stages of the process.
The object of the present invention is to provide an economic and operationally reliable process which enables polyurethane hydrolysates to be separated into refoamable, substantially amine-free polyethers and diamines without the need to apply elaborate analytical control measures.